In the development of modern optical systems, efforts were concentrated initially on developing optical materials of high transmittance and coatings for optical elements that would reduce reflection from element surfaces of the wavelengths to be transmitted by the optical system. It was later recognized that radiation-induced defects in silica and other optical materials could also interfere with the transmittance of desired wavelengths of the radiation. The study of radiation-induced defects in silica based glasses intensified with the advent of fiber optics and the use of photolithography in the manufacture of semi-conductor chips and other electronic devices. Thus, the performance of high purity, highly-transparent glasses can be significantly reduced by absorption bands developing as a result of the inherent incidence of radiation on the glass materials from which the lenses of an optical system may be made.
The principal effect of radiation on highly-transparent glasses of silica or similar optical materials is the creation of molecular or atomic defects such as the creation of electron vacancies or "holes" which may become trapped at certain trapping sites present in glass. Trapping sites might involve atomic vacancies, interstitials sites, strained bonds, multivalent ions, and the like. In addition, high doses of ionizing radiation of sufficiently high energies may serve to create additional trapping sites, particularly an atomic vacancy or an atomic impurity (interstitial). Unpaired electrons also may comprise a radiation induced optical defect. Either unpaired electrons or holes trapped in a silica material may result in optical absorption bands at energies lower than the intrinsic band gap of the material.
Radiation generated defects in high-purity fused silica have been referred to as "oxygen-associated trapped-hole centers" (OHC's). While not wishing to be bound by any one theory, it is postulated that such absorption defects may be caused by electrons trapped at interstitial vacancies within the ordered structure of the silica. Thus, the term "trapped hole centers". Because of the long optical path-lengths inherent in fiber geometry, light absorbing defect centers, even at very low concentrations, can seriously degrade optical fiber performance. Such radiation induced defect centers in high purity, fused silica are also of particular interest in many applications other than fiber optics because silica is a prototype for many glassy radiation transmissive materials. One such application is photolithography which in recent years has facilitated more effective and inexpensive manufacture of semi-conductor devices, such as transistors and integrated circuit wafers.
In the practice of photolithography, a pattern in an optical mask, which corresponds to the features of the integrated circuit to be manufactured, is imaged onto a semi-conductor wafer with radiant energy such as electron beams, gamma rays, X-rays or ultraviolet light. The wafer is coated with a radiation sensitive photoresist composition, which changes chemically during exposure to the radiation over areas determined by the pattern in the mask. After exposure, the photoresist coating is developed, and the semi-conductor wafer is further processed by etching away areas determined by the imaged pattern. The process may be repeated on the wafer until the desired integrated circuit has been fabricated. Such semi-conductor devices are the building blocks of virtually all consumer, industrial and military electronic apparatus today, such as computers, calculators, automated equipment, and communications equipment, including televisions, radios, and stereos.
One radiation source which may be used for conventional photolithography is ultraviolet light which may be provided by an electrode arc lamp generating UV wavelengths of about 260-460 nanometers (nm). In the fabrication of integrated circuits, it is desirable to reduce the size of circuit features as much as possible so that more circuit components may be included on a single integrated circuit wafer of a given size. However, as the resolution of imaged lines approaches one micrometer in width, the conventional UV wavelengths are too long and result in defraction effects which impair effective imaging. This is because at such narrow circuit line widths, the slits allowing the radiation to pass through the mask have dimensions that are relatively close to the wavelength of the UV radiation being used, which significantly influences the behavior of the radiation as it passes through the slits of the mask.
One solution to this problem has been the use of an imaging radiation medium having a shorter wavelength than conventional ultraviolet. While several approaches have been proposed, including the use of X-rays and electron beams, the most promising approach has been the use of deep ultraviolet light having wavelengths in the range of 190-260 nm. Accordingly, a suitable deep UV photoresist known as polymethyl methacrylate (PMMA) has been developed and currently is in use. Molecular bonds of this resist material are broken by exposure to deep UV light so that exposed portions of PMMA coated on a substrate can be removed from substrate by an etching solution or the like. However, one disadvantage of this solution which has kept deep UV from realizing its full potential for providing integrated circuits of greater density has been that the spectral output of optical systems for deep ultraviolet light has deteriorated with age due to the development of a radiation induced absorption band centered at about 215 nm.
In order for an ultraviolet illuminator to be effective for deep UV photolithography, it must expose the photoresist coating to a certain minimum dose per unit area. In addition to producing a high total dose, the source of deep UV radiation must also produce a certain minimum brightness (light flux) for efficient optical transfer to the photoresist area of the wafer. Radiation degradation of the spectral output of a deep UV optical system increases the on-line time required for exposure of the photoresist coating of each wafer. This in turn may result in unacceptable long processing times and consequently low yields per unit time of completed semi-conductor devices. For example, the degradation of an optical system having lenses made of quartz by exposure to deep ultraviolet radiation for a period of about 1,000 hours can double the on-line exposure time required for each semi-conductor wafer. Such degradation may also cause the level of light flux to fall below the minimum irradiance required.
Although high temperature annealing for several hours has been investigated for its effects upon radiation-induced defect centers in high purity fused silicas, these investigations have been for the purpose of developing a hypothetical model of the defect structure and have not suggested a method of heat treatment for commercial application. In addition, significant differences have been observed in the annealing behavior of defect centers in different silica compositions and these differences are not well understood. In some cases, the amount of absorption of certain optical bands has increased with annealing and in others the amount of absorption has decreased with annealing.
It has also been suggested to heat optical elements in instruments such as telescopes, television cameras, periscopes, bombsights and similar sighting and/or recording devices to prevent the condensation of moisture on these optical elements. Such condensation may result in fogging of the optical elements whereby visibility is impaired. Such heating devices have been suggested where the temperature of the optical instrument is lower than the dew point temperature of the ambient atmosphere so as to prevent water condensation on the cooler surfaces of the lenses or other optical elements. Heating components for conventional optical systems include placing electrical resistance heating rings or coatings in direct contact with a lens surface or between different layers of a sandwich-like lens structure. Lenses also have been heated by heating air around or adjacent to the lenses.